Homogeneous Gold Catalysis Using Complexes Recovered from Waste Electronic Equipment

Despite the greater awareness of elemental sustainability and the benefits of the circular economy concept, much waste electrical and electronic equipment (WEEE) is still destined for landfill. Effective methods for valorizing this waste within our society are therefore imperative. In this contribution, two gold(III) complexes obtained as recovery products from WEEE and their anion metathesis products were investigated as homogenous catalysts. These four recovery products were successfully applied as catalysts for the cyclization of propargylic amides and the condensation of acetylacetone with o-iodoaniline. Impressive activity was also observed in the gold-catalyzed reaction between electron-rich arenes (2-methylfuran, 1,3-dimethoxybenzene, and azulene) and α,β-unsaturated carbonyl compounds (methyl vinyl ketone and cyclohexenone). These recovered compounds were also shown to be effective catalysts for the oxidative cross-coupling reaction of aryl silanes and arenes. When employed as Lewis acid catalysts for carbonyl-containing substrates, the WEEE-derived gold complexes could also be recovered at the end of the reaction and reused without loss in catalytic activity, enhancing still further the sustainability of the process. This is the first direct application in homogeneous catalysis of gold recovery products sourced from e-waste.

N-Propargylbenzamide (32 mg, 0.20 mmol), N-pyridine oxide (2.9 mg, 0.030 mmol) and the respective gold complex (0.01 mmol, 0.05 eq) were dissolved in acetonitrile (1.5 mL). The solution was stirred for the amount of time and at the temperature indicated in the main S10 manuscript (Table 1). After the indicated time, the reaction solution was concentrated and the yield quantified by 1 H NMR analysis. Yields were determined by the relative integrals of the amide methylene protons (4.23-4.25 ppm) and the product vinyl proton (6.85 ppm). A pure sample of the cyclized product was isolated following purification by silica gel column chromatography (Rf = 0.25 [n-hexane/EtOAc (7:1)
Yields were determined by the relative integrals of the o-iodoaniline starting material (dd 7.65 ppm) and the product (dd 7.91 ppm). Average yields with standard deviations are provided for each catalyst in the main manuscript (Table 3).
A stirred solution of azulene (
A stirred solution of 1,3-dimethoxybenzene (0.131 mL, 138 mg, 1.00 mmol), methyl vinyl ketone (0.332 mL, 278 mg, 4.00 mmol) and [Au] (0.010 mmol, 1 mol%) in MeCN (4 mL) was stirred at 60 ºC. After 24 h, the reaction mixture was concentrated, the residue diluted in CDCl3 and the yield determined by 1 H NMR analysis, through comparison of the integration of the dialkylated Ar-H singlet at 6.86 ppm, the monoalkylated doublet at 7.02-7.00 ppm and the starting material triplet at 7.21 ppm. Results are shown in Table S1 below:

Oxidative coupling of aryl silanes and arenes
General cross-coupling procedure Scheme S16. Gold(III) catalysed cross-coupling of arenes and aryl silanes.

Crystal data for
Two orientations were identified of ca. 87 and 13% occupancy, their geometries were optimized, the thermal parameters of adjacent atoms were restrained to be similar, and only the atoms of the major occupancy orientation were refined anisotropically (those of the minor occupancy orientation were refined isotropically). S21   Table S4.   Figure S8. The X-ray crystal structure of 2b (thermal ellipsoids drawn at 50% probability).  Table S6.   Figure S9. The X-ray crystal structure of [Me2dazdt(AuBr)2] (thermal ellipsoids drawn at 50% probability). with distances comparable to the intramolecular contacts (3.121 Å), resulting in the formation of a chain of molecules along the direction of the b axis ( Figure S10). S27   Table S8.

Preliminary indicative cost analysis
The gold recycling process employed in this work S4 can be applied to different kinds of WEEE with costs that will vary depending on the configuration of the industrial plant. However, this report demonstrates the use of the gold complexes produced from this process in catalysis, illustrating the potential of joining up the recovery and catalysis processes using the example of SIM cards as the WEEE feedstock. In order to provide a preliminary indicative cost comparison with the cheapest commercial catalyst used for the same catalytic transformations, a calculation has been performed of the cost to prepare the catalyst [AuI2(Me2dazdt)]I3 (1a) from used SIM cards based on small-scale reagent pricing by the major suppliers. A more meaningful cost analysis will need and be performed referring to a higher technology readiness level (TRL) plant than the one here.
The preliminary indicative cost below reveals that, depending on whether the cost of SIM cards is included, the same amount of 1a is 29-55% less than that of commercial AuCl3 (the cheapest of the literature catalysts used in the research). It is often estimated that the scale up of a process reduces costs ten-fold so the costs above for the preparation of 1a would be reduced dramatically on scale up. In comparison, the cost of commercial AuCl3 is already minimized through the savings of large-scale production. This estimate shows that, even unoptimized, small-scale production of catalyst 1a leads to a significantly lower cost than commercial catalysts derived from environmentally-damaging mining.
Scheme S17. Synthesis of Me2dazdt. S2,S4 Energy consumption has been considered but not included, as this is difficult to calculate.

S30
However, an estimation based on the literature S9 suggests that values around 2 kWh would be needed (mainly due to 3h refluxing in toluene). The average cost of one kWh over the last 7 years S10 is € 0.08, rendering the energy cost a negligible € 0.16 ($ 0.16).
Using the cost of preparing Me2dazdt and its use in situ with iodine (as Me2dazdt·2I2), the cost of synthesizing the catalyst [AuI2(Me2dazdt)]I3 (1a) can be calculated: Scheme S18. Synthesis of [AuI2(Me2dazdt)]I3 (1a) see Section 1.2 for experimental details. The estimations above suggest that the cost of 1a is less than half that of commercial AuCl3, which is the cheapest of the literature catalysts used in the research. Additional value generated by valorization of [Cu(NH3)4](SO4) and NiCl2 produced from the SIM cards is not included.

S31
Thirty SIM cards were used to obtain 130 mg of 1a. 1700 used SIM cards are available on ebay for €85 (€0.05 each so 30 would cost €1.50). On scale up, it is anticipated that the cost of used SIM cards would be far lower than €0.05 per card and so this cost would become negligible. Even when including the cost of SIM cars purchased for the purpose of this research, the cost of 1a is 29% less than that of commercial AuCl3.
It is often estimated that the scale up of a process would reduce the cost ten-fold so the costs above for the preparation of 1a would be reduced dramatically on scale up. In comparison, the cost of commercial AuCl3 is already minimized through the savings of large-scale production.